LU-18773 osc: initialize index_orig in osc_brw_prep_request

LU-18773 osc: initialize index_orig in osc_brw_prep_request

This patch initializes index_orig in osc_brw_prep_request() to
fix the following error:

  In function 'osc_brw_prep_request':
  error: 'index_orig' may be used uninitialized in this function
  [-Werror=maybe-uninitialized]
       brwpg->pg->index = index_orig;
       ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~

Change-Id: I97188ea21adfa25950814a04e4f6ffdb9b763712
Test-Parameters: trivial
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58296
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>